Structural assemblies, such as those in the aerospace industry, are often constructed by joining structural members together. During use, these structural assemblies can be subjected to a variety of environmental conditions, temperature variations, load variations, severe acoustic and vibration environments, all of which create mechanical and thermal stresses. The reliability and performance of the structural assemblies under these stresses depends in large part on the material properties of the constituent structural members and any weld joints between the members.
It is commonly recognized that the grain structure of structural members can have an adverse effect on the material properties of the structural members and any weld joints between the members. For example, the grain structure typically associated with conventional aluminum mill products can limit the formability, toughness, weldability, corrosion resistance and strength of structural members formed from these products. As an indication of formability, the typical elongation of AA 2195 aluminum alloys in the T8A3 condition along the longitudinal axis is approximately 11%. The typical elongation of AA 2219 aluminum alloys in the T87 condition along the longitudinal axis is approximately 10%. It is generally believed that the low formability of conventional aluminum mill products, especially in the AA 2195 aluminum alloys, is due to directionality of the grains and poor interlaminar strength. In addition, conventional aluminum mill products joined using common fusion welding techniques typically exhibit weld cracking in the heat affected zone, which can result in relatively weak weld joints. It is generally believed that the poor weldability of conventional aluminum mill products is a result of constitutional liquidation along the grain boundaries as the products are welded.
In addition too conventional aluminum wrought products, metal matrix composites have been implemented in the aerospace industry where high specific strength is required. Metal matrix composites are typically fabricated using powder metallurgy. Powder metallurgy products consist of fine metal powder and ceramic particles compressed together under controlled temperature and pressure (sintering) to produce a billet of material. The high expense associated with the production of fine metal powder and the sintering process makes these powder metallurgy billets less affordable.
In seeking to improve the material properties of structural members constructed of metals and metal alloys, it has been proposed to refine the grain size of the structural members through a process known as “equal angle extrusion.” As illustrated in FIG. 1, equal angle extrusion involves forcing a workpiece 10, using pneumatic or hydraulic pressure, through a die 12 have a 90° bend. In theory, equal angle extrusion crushes the existing grain structure of the workpiece 10 such that the resulting material exiting the extrusion die 12 will exhibit a reduction in grain size. However, difficulties associated with large loads on the die 12 and cracking of the workpiece 10, can adversely affect the properties of the material existing the die. As a result, equal angle extrusion has not been used in large-scale production.
Thus, there remains a need for an apparatus for refining the grain structure of workpieces to thereby provide structural members having improved material properties, such as formability, weldability, toughness, corrosion resistance, and strength. The apparatus should be capable of operating on workpieces that are formed of a variety of metals and metal alloys and that have a variety of configurations. The apparatus also should be cost effective and should be scalable for use in large-scale production operations.